1. Field of the Invention
The present invention relates to a device for testing material on a test object having at least electrically conducting and ferromagnetic material parts, the test object having at least one technical surface with at least one electromagnetic ultrasonic transducer (EMUS) provided with at least one magnet and at least one eddy current coil.
2. Description of the Prior Art
Electromagnetic ultrasonic transducers are used in a known manner for the purpose of non-destructive material testing and measurement of test objects comprising electrically conducting materials which moreover possess ferromagnetic properties.
Basically electromagnetic ultrasonic transducers can be differentiated into two types: on the one hand, those with which produce so-called horizontally polarized shear waves which are able to propagate inside the test object predominantly parallel to the coupling-in surface; and on the other hand, ultrasonic transducers for generating in the test object so-called freely propagating ultrasonic waves preferably propagating inside the test object perpendicular to the coupling-in surface. In both instances, excitation of ultrasonic waves inside a test object results from the occurrence of magnetostriction and Lorenz forces inside the test object material, which can be generated by the presence of a temporally largely constant magnetic field overlapping with an electromagnetic alternating field generated by an electro-magnetic alternating current.
A typical setup for exciting ultrasonic waves according to the so-called EMUS principle is shown in FIGS. 8a and 8b. Common EMUS transducers 3 comprise a permanent magnet 1 and an eddy current coil 2, which are designed as one unit for joint handling. Usually the eddy current coil 2 is designed as a rectangular flat coil or a spiral flat coil each of which have an electrically conductive strip and is attached to a magnetic pole side of the permanent magnet 1 in such a manner that a permanent magnetic field passes vertically through the coil 2. If the aforementioned EMUS transducer 3 is placed on an electrically conducting ferromagnetic test object 4, the permanent magnetic field overlaps inside the test object with an eddy current field generated by the eddy current coil, on the one hand, generating magnetostrictive effects due to the overlapping of the magnetic field components of the eddy current field with the permanent magnetic field entering vertically through the surface of the test object and, on the other hand generating the Lorenz forces due to the eddy currents induced in the test object, which then generate pressure waves occurring normally in relation to the surface of the test object as well as radially polarized shear waves capable of propagating as ultrasonic waves inside the test object. Both types of ultrasonic waves, that is the ultrasonic waves propagating normally in relation to the surface of the test object and ultrasonic waves propagating in longitudinal direction to the surface of the test object due to radially polarized shear waves are suited according to the state of the art for testing faults, for example detecting cracks inside the test object, as well as for measuring the thickness of the wall of the test object.
Since in use eddy current coils are very sensitive to outside mechanical influences, the eddy current coils must principally be protected against mechanical wear, which is difficult in particular due to the fact that in ferromagnetic test objects the eddy current coil located between the permanent magnet and the test object is pressed onto the surface of the test object by the magnetic forces of attraction and is therefore subject to considerable fretting.
In this context, German Patent 35 11 076 A1 describes a test pig for electromagnetic testing of the walls of steel pipes, such as, for example as part of nondestructive testing of wall weaknesses due to rusting of the pipe walls. A pig, which is described in detail therein, is provided with electromagnets, which are distributed uniformly around the circumference, each comprising two measuring heads which are axially aligned to each other, a yoke connecting the measuring heads and a magnetizing coil on the measuring heads, with the field of each electromagnet running parallel to the center axis of the pipe. For ultrasonic measurement, an eddy current coil, to which are applied strong and very rapidly rising current pulses, is disposed directly at least on one of the poles, and the measuring heads. The pipes of pipelines are provided with circumferential seams at the adjoining parts of two adjacent pipe pieces. When the above briefly described test pig runs over the seams during continuous inspection, the circumferential seams subject the electromagnetic transducer to impacts which, moreover, are markedly intensified by the magnetic forces prevailing between the electromagnets and the wall of the pipes. The previously described fretting and the additional impacts to the electromagnetic ultrasonic transducer, in particular to the eddy current coil, lead to a short lifetime of the EMUS transducer, which needs to be addressed.
Although fretting can be reduced by decreasing the magnetic forces of attraction prevailing between the EMUS transducer and the to-be-inspected test object, for example by decreasing the magnetic field induction, this measurement would also immediately lead to distinctly diminishing the EMUS transducer's efficiency, that is force density induced to generate ultrasound inside the test object reduces in the same way, due to which the detection sensitivity in receiving scattered or reflected ultrasonic waves diminishes to the same extent.
Japanese Patent 111 33 003 describes a device for inspecting material using ultrasound which is suited in particular for inspecting the material of pipes. According to claim 4 therein, the device comprises single permanent magnets which are arranged to form a ring of segments with an outer and an inner circumferential edge. The adjacent permanent magnets have opposite magnetic poles at the outer and inner circumferential edge. Disposed in windings on the outer circumferential edge of this ring is an electrical strip conductor of at least one eddy current coil. The device is introduced in operation into a pipe that the outer circumferential edge with the strip conductors slides along the inner wall of the pipe, leading to corresponding fretting on the strip conductors.
U.S. Pat. No. 4,898,034 describes a device for testing hot materials, such as metals and ceramics, using ultrasound. An embodiment uses an agent made of zircon which is in contact with the hot material to be examined. Furthermore, a coupling medium (borax) is in contact with the hot material and the zircon agent. The zircon agent and the coupling medium receives ultrasonic waves propagating from the hot material through the coupling medium and the zircon agent. In the embodiment shown in FIG. 1 of U.S. Pat. No. 4,898,034, the zircon agent is designed as a ring with an outer and an inner circumferential edge. In operation, the outer circumferential edge of the ring is rolled over the hot material to be examined. A lever, which is attached to the rotational axis of the zircon ring, holds the ultrasound transmitter constantly as shown in downward perpendicular position. In this manner the ultrasound transmitter including the eddy current coil attached to it is pressed against the inner circumferential edge of the ring, leading once again to fretting of the ultrasound transmitter.